Marine navigation devices often include a display that shows a map of at least a portion of a body of water in the vicinity of the current geolocation of a marine vessel with which the marine navigation device is being utilized. The map may include a representation of the terrain or soil at the bottom of the water with contour lines which indicate locations of the terrain that are at the same depth. Marine navigation devices display contour lines associated with depths of a plurality of geographic areas associated with a body of water. The displayed contour lines are presented in a higher density for areas associated with, large gradients or slopes of the underwater terrain, while flatter underwater regions result in a lower contour line density. Conventional marine navigation devices may remove an entire contour line to reduce the density of contour lines presented on a display.